Ceramics are attractive candidate materials for wear applications owing to their excellent properties of thermal stability, high elasticity, hardness, resistance to chemical corrosion and low inertia. Monolithic oxide, nitride and carbide ceramics, as Al.sub.2 O.sub.3, ZrO.sub.2, Si.sub.3 N.sub.4, SiC and others have been recognized as potential materials for use in structural applications such as seal rings, valve seats, dies for extrusion, guides, valve tram components and bearing parts of cylinder liners (Czichos H., Woydt M., Klaffke D. and Lexow J., Materiaux et Techniques, 1989).
The major shortcoming of the materials is their low fracture toughness, resulting in inherently brittle components. In recent years considerable effort has been devoted world-wide to improving ceramic fracture toughness, primarily for structural engineering applications, by incorporating additional phases into the base materials generally in the form of fibers, whiskers or particles (Evans A. G. and Cannon R. M., Acta Metall 34 (1986) 761). Very few studies have considered the development of Ceramic Matrix Composite (CMC) materials for tribological applications and most of these are focused on whisker reinforced ceramics (Yust C. S., Leitnaker, J. M. and Devore, C. E., Wear 122 (1988) 156, Yust, C. S. and Devore, C. E., Tribol Trans 34 (1991), Liu H., F E M E and Cheng H. S., J. Am Ceram Soc 74 (1991) 2224). Nevertheless, the improvement of fracture toughness of ceramics designed for wear applications is of considerable industrial importance and the tailoring of the combined properties of the component phases offers an excellent opportunity to optimize the frictional and wear properties of the composite.
Wear damage in ceramic-metal couples in dry sliding conditions arises from mechanical and tribo-chemical interactions between the touching faces. Mechanical abrasive wear results from impact fracture of microscale features of the ceramic and the physical removal of particles from the ceramic surface. These particles may become trapped between the wear surfaces and further contribute to wear damage of the couple. Tribo-chemical, adhesive wear arises from cold welding of asperity junctions in the contact faces. Further, sliding leads to the fracture of these junctions in one or other of the contacting materials. In the case of ceramic/metal pairs, adhesive wear can result from high local surface temperatures. The location of the subsequent fracture will depend upon the particular materials involved. The intrinsic strength of engineering ceramic materials confers a high degree of wear resistance. Unless the ceramic exhibits weakness or brittleness in the microscale of the weld zone, the wear debris will contain a reasonable proportion of metallic component. Such debris can re-adhere to the contacting surfaces leading to the formation of an adherent transfer film of metal on the ceramic. Subsequent contact with the metal counter body may then lead to adhesive wear as the bulk metal adheres to the transfer film.